What do you think of people who rarely smile or look serious alot of the time?

What do u think is going on in there heads, do you like them or do they bother you. If they were in your cl or in your vicinity, would you venture to talk to them? or would you consider them scary and unfriendly? i was wondering because it seems that sometimes people dont take the time to get to know some people because they never make an attempt and it results in the person being alienated and an outsider which is lonely. What would you choose to do, do you talk to those sort to discover that they are practically different people or what you thought of them as?